If you experience any major fluid loss (hemorrhage/diarrhea) there will be a decrease in fluid volume and therefore blood pressure.
To compensate your body activates a cascade of events to correct this.
1) Your perfusion to your kidney's decreases from less blood volume
2) In response it release renin which cleaves angiotenisin into angiotenisin II ( a major vasoconstrictor)
3) Angiotensisin II will cause ADH and aldosterone lvls to increase
4) Both ADH and aldosterone act to increase fluid absorption in your kidneys.
5) The resultant increase in fluid will raise your venous return ( more fluid = more pressure according to bernoulli's principle)
6)As a result of the vasoconstriction of peripheral vessels and increase in fluid volume your BP returns to normal and renin is no longer released by the "now" adequately perfused kidney.
